Protective system for electric cables



Dec. 25, 1928. 1,696226 F. DOUCHET s PROTECTIVE SYSTE FOR ELECTRICCABLES Fiid April 12. 1927 R, I 4%* 1! I in ;II and III of thepowercable.

Patented Dec. 25, 1928.

UNITED STATES PATENT OFFICE.

FLORIMOND DOUCHET, OF ARIS, FRANCE, ASSIGNOR TO FORGES &7 ATELIEBS DECON STRUCTIONS ELECTRIQUES DE JEUMONT SOCIT ANONYME, OI' PABIS, FRANCE,A.

FRENCE JONT-STOCK COMPANY.

PBOTECTIVE SYSTEM FOR ELECTRIC CABLES.

Application filed April 12, 1927, Serial Ne. 183 ,152, and in France May7, 1826.

This invention relates to electric cut-out or protective systems withpilot conductors.

Diferential select-ive protective systems for cables transmittingelcctric power are in general based upon the following principle Atransformer is connected at each end of each of the conductors in thesection of the power cable to be cut out upon occurrence of a fault, thesecondaries of the transformers belonging to the same conductor beingconnected together by a pilot wire; the several pilot wires may form alow-tension pilot cabl arranged externally of the power transnissioncable. The transforner secondaries connected to the same pilot Wire tendto circulate currents of opposite direction thercin.

When the power cable section considered is undamaged, the intensitics ofthe Currents circulating in the conductors of the said section areequalat the two extremities; consequently, the intensities of theCurrents which the secondaries of the transformcrsplaced at theextremities of the section in question tend to create are also equal butof op-positc directions, so that their resultant 'is nil and thereforeno cunt circulates in the pilot wires.

In case of a fault or breakdown, the intensities of the Currents at thetwo extremities of the section to be cut out or isolated being no longerequal, a resultant secondary current circulates in the pilot cable andtherefore flows through the windings of the relays placed at the twoends or terminals of this section. At this moment the said terminalrelays are actuated and the damaged section is thereby cut out orisolated.

i Referring to the accompanying drawings, -Figure 1 representsdiagrammatically an arrangement of this kind applied to an elcctricpower transmission by three phase current. I, II and III designate theconducting cores of the power cable, while 11, 12 and 13 are theconductors of the pilot cable; T T

T T /--T T represent the transformers arran ed respectively at the twoends 'of the con uctersi I, II? and III of the power cable sectionconsidered. I

The arrows A A A indicate the direction of the currents flowing intheconductors I,

The arrows 13 i, and i( 73,' z',' indicate the end of the section.

R R R represent the relay windings connected to the pilot cable at theopposite end of the section under consideration.

In the ordinary arrangement, on the one hand the secondaries of thetransformers T T T and on the other hand those of the transformers T T'T are connected together star-wise, that is to say, the threeextremities of their windings Situated on the same side are connectedtogether, the poin of connection usually terrned the neutral point beingat the potential of the earth.

An arrangement'according to the diagram represented in Figure 1 presentsthe advantage that the pilot cable comprises only three wires of smallsection.

In other arrangements, based upon the same principle, the terminaltransformers are shunted by low resistances or low impedances in such away as to provide at the two extremities under consideration anequilibrium of voltage instead of an equilibriun of current intensity. i

Selective and difl'erential cut out or protective devices of the kindsabove described are well known and have been applied in a large numberof installations.

Naturally, the power transmission cable to be protected may be athree-pole cable for high tension or a single-pole cable system as isused for Very high tensions. r i

The pilot cable is always one in which there arise in normal workingonly low tension currents. However, in certain applications,

. when a fault has, occurred in the power cable,

it has been ascertained that-the pilot cable has been damaged as if ithas been subjectecl f tions traversed during a very short period of timeby a current of which the intensity is given ble, thereby determiningtheir breakdown or b an equation of the form A=V/Z, where X designatesthe voltage with relation to earth at the moment of the fault '(V might'`likewise designate the voltage between conductors in the case of abreakdown between the conductors), and Z is the impedance comprisedbetween the extremity of the conductor I and the position of the fault.

Since the cables usually posses ver low impedance, the current A is ingener very great in high tension cables.

The current A is one which oscillates very rapidly, the value of thepulsation being determined by the electrical characteristics of thedamaged power cable and of the network. It results that this currentcreates a magnetie flux with rapid variations in the space whichsurrounds the conductor I. A part ofthis magnetic flux traverses thecircuit constituted either by a pilot wire and earth or by two pilotwires.

For unit current in the power conductor, the flux passing through thesecond circuit considered may be designated by M.

This coeflicient M would be'what is known in electro-technicsas thecoeflicient of mutual induction, if the variations of current in theconductor I were not sufliciently great -to create antagonistic fiuxesand eddy Currents in the surrounding metallic masses. The coefl icient Mmay therefore not be constant but may depend on the current A and on theva riation of this latter.

The circuit constituted by one pilot wire and the earth or again by twoadjace'nt pilot wires, being traversed by an electro-magnetic fiuxhaving rapid variations of the form'M A, is subjected to the action ofan electromotive force of instantaneous value given by the derivativewith respect to time of the precedng expression, viz:

. tive force to attain considerable magnitude and for a very highpotential difference U to occur between the conductor of the pilot ca-.damage. The said electro-motive force may moreover not be constantalong the ilot cable but may create a stationary wave o voltage;

* A diaphragm of this phenomenon is given in Figure 2 of the acconpaning drawing, where there are represented on y the elements alfecting theconductor I.

The present invention'has for its objects to avoid the manifestation ofthese high potential difierences between pilot wires by electromagneticcoupling wth the power cable, and thereby to su press the cause ofdamage already discussed? In order to attain these objects, it isnecessa'y'to red ce the portion passing through the' ilot wires, of theflux produced by the con uctor I. For this purpose it may be soughteither to diminish the value of the coeflieent of mutual induction atlow frequency, or to increase the antagonistic fluxes at hi hfrequ'encies.

1`he present invention has the advantage of fulfilling simultaneouslyboth those ob ects by consttuting the elementary ci'cuit of the pilotcable correspondin to one phase of the power cable by means o twoconductors, one

of these conductors preferably surrounding the other so that the twoconductors may therefore be arranged concentrically.

F igure 3 of the drawing represents diagrammatically by way of example asystem according to the invention in its application to one phase only.

'T and T are the balancing transformers for the two ends of the sectionto be protected.

R and R are the windings of the corresponding relays which ma be placedat any convenient point in the pi ot circut.

11 and 11' are the concentric conductors of the pilot cable corres ndingto the conductor I of the .power cab e; these conductors are connectedin series and in their circuit there are included the secondaries of thetransformers T and T as well as the relay windings R and-R i It is quiteevident that in such an arrangement the equilibrium of vol'tage or ofcurrent, characteristic of the diflerential protective system, has notbeen modified in its principle; but it is equall clear that the portionof the flux produced y the' conductor I, which normally 'acts belectro-magnetic induction upon theron uctor 11 in the ordinary ar-''angement, is considerably weakened by the presence of the conductor 11' surrounding the conductor 11.

The area between the conductors 11 and 11' embraced by the flux due tothe conduc'- tor I is in fact a minimum, so that no dangerous rise ofvoltage in the conductor 11 with reference to the conductor 11' is to befeared.

In order to prevent a rise of voltage between the conductor 11' andearth, it suflices to fix its potential by a connection to earth or tothe lead sheath' of the pilot cable.

For reasons of faci 'ty of Construction, it will be clear that the twoconductors 11, 11' will preferably be concentric as has already been'explained The conductor 11' may be construct'ed in any desired mannerbut must be constituted of a material which is a very good conductor ofelectricity, so as increase as far as poible the antagonistc Currentscreated in'this sheath or armouring by the rapidly varying flux.

This sheath 11' may for example be formed by a metal tube or a conductorcomposed of metal wires arranged coneentrically of the central conductoror again by a meta band or nected mesme one or more metallized layerswound hclicaL ly around the central conductor.

. What I claim is 1. A protective system for electric cables, comprisingapair of transformers, said transformers having their primaries includedin the circuit of one conductor of the cablc to be protected, and havingtheir secondarics connected in opposition by a pilot circuit, said pilotcircuit conprisng two conductors, one surrounding the other.

2. A protective system for elcctric cables,

comprising a pair of transforners arranged' at opposite ends of asection of cable to be protected, said transformers having theirprimaries included in the circuit of one conductor of said section ofcable, and having their secondaries connected in opposition by a pilotcircuit, said pilot circuit conprising two conductors, one surroundingthe other for substantially the length of `said section of cable. y

3. A protective s sten for electric cables, comprisng a pair otransformers, said transformers having their rimaries included in thecircuit of one con uctor of the cable to be protected, andhaving theirsecondaries connected in opposition by a pilot circuit, said pilotcircuit comprising a concentric twowire system.

4. 'A rotective system for electric cables, comprisng a pair oftransformers, said transformers having their primaries included in thecircuit of one conductor of the cable to be protected, and having theirsecondaries conin opposition by a pilot circuit, said "pilot circuitcomprising two conductors, one surrounding the other, the outer one ofsaid conductors being of highly conductive material and foring anarmouring around the 4 inner one of said conductors.

5. In a ilot wire protective system for electric cab es, the combiationof a pair of balancing transformers connected to one conductor of saidcable, with a two-wire pilot circuit, said pilot circuit includin thesecondaries of said transformers and t ie operating windings of theprotective relays for said conductor, and the two wires of said pilotcircuit being arranged concentrically 60 for substantially the whole oftheir length.

6. In a pilot wire protcctive system for electric cables, thecombination of a pair of' balancing transformers connected to oppositeends of a section of conductor to be protected, relays connected to thesecondaries of said transformers, saidrelays adaptedto isolate saidsection of conductor, and a twowire pilot circuit connecting saidsecondaries and said relays, said ilotcircuit having its two wires inconcentrc relation for substantially the whole of their length.

7. A protective system for polyphase electric power cables, comprising apair of transformers for each phase, said pair of trans formers beingconnected to opposite ends of a section of ,conductor of said phase, anda pilot circuit for 'each phase, said pilot crcut including thesecondaries of a pair of transformers and the windings of the protective7 apparatus for said conductor, each pilot circuit consisting of atwo-wire system having its conductors arranged concentrically forsubstantially the whole of their length.

In testimony whereof I hereunto aflix my signature.

FLORIMOND DOUCHET.

CERTIFICATE OF CORRECTION.

Pate't, 11696326.

Grnted December 25, me, to

FLORIMOND DOUCHET.

lt ie herehy certified that error appears in the printed specifictiou ofthe above numbered patent requiring correction as ollows: Page 2, line9, tor the miepelled word "poeses"red "possess"; ductors' lie 54, for"diphrgn" read "digram";

line`49, for "coductor" read "conlin 73. for "those" read "these", andlinos 94 and 95, strike out the word "and"; and tht the said LettersPtent should be read with these corrections.

therein that the same y confor to the record ot the case in the PtentOffice.

Signed d seled this 16th dy of April, A. D. 1929.

M. J. Moore, Acting Commisioner of Patents.

